Coated rolling element bearing

ABSTRACT

A rolling element bearing, comprises an inner ring, an outer ring and rolling elements which are in rolling contact with the raceways of the inner ring and the outer ring. The rolling elements and/or the raceway ( 2 ) of at least one of the rings is coated with a metal-mixed diamond-like carbon layer ( 8 ).

The invention is related to the field of rolling element bearings.Rolling element bearings are applied in various environments. As long aslubrication is guaranteed, their normal design life can generally beobtained without problems.

Under poor or marginal lubrication conditions however, the behaviour ofrolling element bearings is less favourable. In such cases, it may proveto be impossible to form a lubricant film, resulting in directmetal-to-metal contact of the rolling elements and the rings, and thusleading to adhesive wear, such as fretting, and premature failure.

Also, rolling element bearings which are exposed to periods of vibratoryor oscillatory motions while under static load without rotation of thebearing often show a phenomenon known as “False-Brinelling”, which againcan limit bearing life.

As a consequence of these phenomena, the service life under poorlubrication conditions is greatly impaired. Attempts have been made toalleviate this problem by applying hard coatings onto the bearingcomponents, e.g. comprising carbide and nitride coatings (e.g. TiN, TiC,CrN), as well as solid lubricants such as molybdenum -di-sulphide.

None of these attempts however has yielded a significant improvement.Although nitride and carbide coatings have high hardness levels and dooffer a good abrasive wear resistance and high hardness, their frictioncoefficients are high. These high friction coefficients in combinationwith high hardness leads to abrasive wear of the counterface, i.e. of aring or rolling element.

Furthermore, the high adhesion of TiC, TiN and CrN coatings is onlyattained with relatively high coating temperatures of the order of 400°C. Such coatings are widely used for cutting tools where the substratematerial is made from highly alloyed high speed steel which retainshardness at temperatures of 500 to 540° C., due to the phenomenon ofsecondary hardening. Such coating temperatures would result in excessivesoftening and dimensional changes with the lower alloyed steels that arecommonly used in rolling element bearings.

According to U.S. Pat. No. 5,108,813, normal diamond-like carboncoatings can be applied in sliding bearings. A diamond-like carboncoating is defined as a layer of amorphous hydrogenated carbonconsisting of both sp3 and sp2 hybridised carbon bonds. While in slidingbearings the adhesion of a diamond-like carbon coating to the substrateis sufficient, in rolling element bearings such coatings flake off as aresult of the much higher contact stresses in a rolling element bearing.

The object of the invention is therefore to provide a rolling elementbearing which has an increased service life under conditions wherelubrication is poor or absent. This object is achieved in that therolling elements and/or the raceway of at least one of the rings iscoated with a metal mixed diamond-like carbon coating.

The metal mixed diamond-like carbon coating according to the inventionoffers both low friction and high wear resistance, even under the highcontact stress levels encountered in rolling element bearings (suchstresses can be up to 3 or 4 GPa). The diamond-like carbon coatingaccording to the invention, mixed with carbide forming metal atoms iscapable of supporting these high contact stress levels without flakingwithin itself or from the substrate, when the coating structure isoptimized to give high adhesion.

Thus, the coating in question is suitable for application for rollingelement bearings operating under dry conditions or where lubrication ispoor or even absent.

According to the preferred embodiment, the metal mixeddiamond-like-carbon coating is formed on a metallic interlayer of Cr, W,Mo, or Al, which is predeposited onto the substrate surface. Thereshould be a gradual transition of the metallic interlayer into the metalmixed coating. The diamond-like carbon coating, applied to the metallicinterlayer, also contains one or more metallic carbide forming elementssuch as W, Mo or Ti.

The metal mixed diamond-like-carbon part of the coating may be composedof alternating parallel layers of predominantly diamond-like-carbon, butcontaining some metal carbide, and predominantly metal carbide, butcontaining some diamond-like-carbon, each of the order of 1 to 50nanometre thickness.

Alternatively, the metal carbide can be in the form of clustersinterspersed throughout a layer of diamond-like-carbon Such layersconsists of a matrix of predominantly diamond-like carbon with adispension of particles of metal carbide. Said particles can have aclustered morphology giving regions (clusters) of metal carbide up to100 nm in extent. The clusters may be composed of carbides of one ormore metals.

The thickness of the metal mixed diamond-like-carbon coating can be upto 10 μm. The multi-layer structure of the coating gives rise to reducedresidual stresses in the coating and therefore enhanced adhesioncompared to conventional single layer diamond-like-carbon coatings withor without a metallic interlayer.

A further important feature of the coating is the possibility todecrease the metallic carbide forming element content towards thesurface during the coating process, thereby increasing the diamond likecharacter of the outer layers of the coating. This still results in highadhesion to the substrate but at the same time also allows a significantincrease in the surface hardness as the outer layer of the coating isnow closer to pure diamond like carbon. In this case, the low frictionand increased hardness gives benefits of increased wear resistance,whilst retaining high adhesion necessary for rolling bearingapplications.

Also, there can be a gradual increase in graphite content towards thecoating surface. This has the benefit of further reducing the frictioncoefficient. Of course, both a reduction of the metallic carbide formingelement, and an increase in the graphite content towards the coatingsurface can be achieved.

The metal mixed diamond like carbon coating may also contain metallic Nior possibly Fe as a means of reducing cost of the carbide formingelement source material.

Furthermore the metal-mixed diamond-like-carbon coating can be appliedat a coating temperature of <240° C., with the high adhesion levelsnecessary for rolling element bearings. In this respect temperaturesensitive low alloy bearing steels in the hardened and temperedcondition can be coated in such a way that microstructural or phasechanges of the substrate material do not occur, with the result thatthere are no permanent irreversible size changes of the coated bearingcomponents. This allows assembly of rings and/or rolling elements withstandard components and avoids the need of matching in order to obtainthe desired bearing precision and internal clearance.

In addition the low coating temperature ensures that the substratehardness and fatigue properties of temperature sensitive low alloybearing steels are not impaired and that standard static and dynamiccapacity ratings for the bearing still apply. Nevertheless, the coatingcan also be applied to high alloy steels, such as martensitic stainlesssteels and high speed steels.

The invention will be explained further with reference to the examplesshown in the figures.

FIG. 1 shows a first embodiment of the invention.

FIG. 2 shows a second embodiment of the invention.

FIG. 1 shows a cross-section to the outer layers of the rolling elementor raceway of a ring of a rolling element bearing. The full outer parthas been indicated in its entirety by reference No. 1, and has beenapplied to a substrate 2, e.g. bearing steel.

Said full outer part 1 comprises an intermediate metallic interlayer 3,which has been applied directly onto the substrate 2, a transition zone4 which follows the intermediate metallic interlayer 3, and ametal-mixed diamond-like carbon coating 8. The transition zone 4comprises an area wherein a multi-layer transition is obtained, withalternating metal-carbide layers 5 and metallic layers 6. Subsequently asmooth transition area 7 is obtained, into the multi-layer structure 8.The transition area 7 provides a smooth transition from metal-carbide toa mixture of metal-carbide and diamond-like carbon. Said multi-layerstructure 8 comprises alternating metal carbidelayers 9 and diamond-likecarbon layers 10.

FIG. 2 shows a cross-section to an alternative embodiment, also of theouter layers of a rolling element as the raceway of a ring of a rollingelement bearing.

The full coating 11 comprises a matrix 12 of predominantly diamond-likecarbon, with a dispersion of metal carbide particles 13. Such particlesor clusters 13 of metal carbide may have a maximum dimension of 100 nm.

The substrate 2, interlayer 3 and transition zone 4 are similar to thecorresponding ones in the embodiment of FIG. 1.

What is claimed is:
 1. A rolling element bearing, comprising an innerring, an outer ring and rolling elements which are in rolling contactwith the raceways of the inner ring and the outer ring, wherein at leastone of the rolling elements and the raceway of at least one of the ringsis coated with a metal-mixed diamond-like carbon coating comprisingalternating layers of predominantly diamond-like carbon, but containingsome metal carbide, and layers of predominantly metal carbide, butcontaining some of the diamond-like carbon.
 2. The rolling elementbearing according to claim 1, and comprising ancillary components whichare in contact with at least one of the rings, and rolling elements andanother of said components, wherein at least one of said ancillarycomponents is coated with a metal-mixed diamond-like-carbon coatingwherein the metal of the metal-mixed diamond-like carbon is in the formof clusters interspersed throughout a matrix of diamond-like carbon. 3.The rolling element bearing according to claim 2, wherein the ancillarycomponent is a cage.
 4. The rolling element bearing according to claim2, wherein the ancillary component is at least one seal.
 5. The rollingelement bearing according to claim 2, wherein the ancillary component isat least one shield.
 6. The rolling element bearing according to claim2, wherein the ancillary component is a guide ring.
 7. The rollingelement bearing according to claim 1 wherein the thickness of themetal-mixed diamond-like-carbon coating is up to 10 μm.
 8. The rollingelement bearing according to claim 1 wherein the metal-mixeddiamond-like-carbon coating is coated onto a metallic intermediatelayer.
 9. The rolling element bearing according to claim 8, wherein themetal-mixed diamond-like carbon coating is coated onto a metallicintermediate layer of Cr, W, Mho or Al.
 10. The rolling element bearingaccording to claim 8, wherein between the metal-mixeddiamond-like-carbon coating and the intermediate metallic layer, thereis a transition zone.
 11. The rolling element bearing according to claim1 wherein the metal-containing diamond-like-carbon coating has beenapplied by coating at a temperature of <240° C.
 12. The rolling elementbearing according to claim 1 where there is a gradual reduction of themetallic carbide forming element content towards the coating surfacesuch that the outer regions of the coating become more diamond likecarbon in nature.
 13. The rolling element bearing according to claim 12,wherein the carbide forming element is selected from the groupconsisting of W, Mo and Ti.
 14. The rolling element bearing according toclaim 1 wherein there is a gradual increase in graphite content towardsthe coating surface.
 15. The rolling element bearing according to claim1 wherein the metal-mixed diamond-like-carbon-coating contains Ni. 16.The rolling element bearing according to claim 1 wherein themetal-mixed-diamond-like coating comprises alternating layers ofpredominately diamond-like-carbon, and layers of predominantly metalcarbide.
 17. The rolling element bearing according to claim 1 whereinmetal is in the form of clusters interspersed throughout a matrix ofdiamond-like-carbon.
 18. The rolling element bearing according to claim17, wherein the maximum extent of the clusters is 100 nm.